Population genetics · USMLE Step 1

A small group leaves a large population and establishes a new isolated community. After generations, a previously rare allele becomes more common. Which effect explains this?

  1. Gene flow
  2. Founder effect
  3. Genetic drift
  4. Directional selection
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Correct answer: Founder effect

The founder effect occurs when a small group establishes a new population with different allele frequencies from the original. Rare alleles may become common. This is distinct from gene flow, which mixes populations.

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